1. Field of the Invention
The present invention relates to a lens sheet used in a shadow mask for a display device such as a transmission screen for a projection TV, a Plasma Display Panel (PDP), a Liquid Crystal Display (LCD) and so on; and a producing method of the lens sheet. In particular, the invention relates to a method of forming a shielding layer with simple in a lens sheet having a fine pitch of 0.3 mm or less required for a pixel motor mode projection TV screen using a liquid crystal panel. Further, the invention relates to a method of forming a shielding layer in a fresnel lens which can remarkably improve contrast by reducing stray light.
2. Description of the Related Art
A transmission screen used in a projection TV is generally an assembly of a fresnel lens sheet and a lenticular lens sheet. In the projection TV adopting a CRT as a light engine (hereinafter will be referred to as CRT type projection TV), the lenticular lens sheet 11 is provided with convex cylindrical lenses 12 and 13 at both planes for radiation/incidence of image light as shown in FIG. 11. The lenticular lens sheet 11 functions to compensate the color difference in RGB 3-colors from the CRT as well as enlarge the angular field of image light incident to the lenticular lens sheet from the fresnel lens sheet.
Further, the lenticular lens sheet 11 has protrusions 14 each formed in boundaries of the convex cylindrical lenses 13 and a light-absorbing shielding layer 15 each formed on the tops of the protrusions 14. The light shielding layer 15, which is generally called black stripes, can prevent reflection of any external light from an observer as well as improve the contrast of image light radiated from the lenticular lens sheet 11.
The above described lenticular lens sheet 11 is formed by pressing a transparent thermoplastic resin sheet. Alternatively, the transparent thermoplastic resin sheet undergoes molding as well as extrusion (hereinafter will be referred to as extrusion molding) to form the convex cylindrical lenses 12 and 13 and the protrusions 14. The formed lenticular lens sheet is provided with the light shielding layer 15 by selectively printing light-absorbing colored pigment only on the protrusions 14 according to various printing methods such as screen printing and gravure printing.
In forming the above described lenticular lens sheet 11, location matching between in front and rear sides, which are the convex cylindrical lenses 12 of the incidence plane and the convex cylindrical lenses 13 and the shielding layer 15 of the radiation plane, is important. If axes of the both lenses in the front and rear sides do not have positional relation corresponding to the light incident to the convex lenses on the incidence plane, the shielding layer 15 blocks radiation light to degrade the lightness of radiation thereby deteriorating screen characteristics.
The lenticular lens sheet for the CRT type projection TV adopts the above described pressing or extrusion molding so that the location is performed in a relatively easy manner at the front and rear sides of the sheet. Also, the forming can be so performed to impart sufficient screen characteristics to the sheet. Alternatively, the lenticular lens sheet may adopt a microlens array sheet where microlenses are arranged. The microlens array sheet functions to widen the angular field of the projection TV not only horizontally but also vertically. Hereinafter in the application, the convex cylindrical lens, the microlens and the fresnel lens will be generally referred to as “lens” and the lenticular lens sheet, the microlens array sheet and the fresnel lens sheet will be generally referred to as “lens sheet.”
In order to obtain high quality and high definition of images, the projection TV requires to adopt an LCD projector, a Digital Micromirror Device (DMD) and so on having smaller pixels as a light engine (i.e. pixel type projection TV) rather than a CRT having larger pixels. However, when the related lens sheet for the CRT type projection TV is diverted into the pixel type projection TV, interference fringes (i.e. moire) are created as a drawback since the size of the pixels projected on the screen is similar to the pitch of lenses in the lens sheet. Therefore, in the lens sheet for the pixel type projection TV, the lenses are required to have a fine pitch, in particular at 0.3 mm or smaller, in order to prevent the moire between the lenses and the pixels projected on the screen.
It is necessary to mold both sides of the lens sheet in the related method for forming a shielding portion by printing convex or concave regions in respect to the concave and convex pattern of the radiation plane in the lenticular lens sheet. However, in the pressing or extrusion molding, it is extremely difficult to form the fine-pitched lenticular lens sheet through the extrusion molding on both sides by the following reason. The fine pitch tends to degrade the molding rate of lens or incur axial deviation at the front and rear sides.
Accordingly, it has been often considered a technique for radiating light from the side of a related lens plane to activate a photosensitive layer applied to the opposite side in order to install a shielding layer only in a non-radiating portion which will not radiate light. Japanese Laid-Open Patent Application No. S59-121033 and H9-120101 disclose methods of blacking a tacky labyrinthine light portion by radiating light from the lens plane side to a positive photosensitive resin layer applied to the opposite side of the lens plane to cure the resin layer, and then printing powder colorant or black pigment or attaching a transfer film printed with black pigment to the photosensitive resin layer. However, those methods have problems that an additional process step is necessary to remove the colorant or pigment from the light condensing portion and a large amount of substrate film used as the transfer film heavily burdens the environment with waste materials. Further, it is difficult to completely remove the pigment or colorant from the light condensing portion. Also, Japanese Laid-Open Patent Application No. 2000-2802 discloses a method of providing a shielding layer by controlling hydrophilicity on the surface with photo-catalyst. However, since the light condensing portion of the lens becomes hydrophilic according to this method, the whole surface of the light condensing portion is printed after applying water. So, an additional process step is required to remove ink from a portion where water is applied thereby sophisticating the whole process.
In the meantime, as shown in FIG. 7A, a typical fresnel lens sheet 6 includes a flat incidence plane 7 of image light and a radiation plane 8 constituted of fresnel lens faces 9 and rising faces 10. As shown in FIG. 7B, image light 21 from a light engine 20 is incident on the fresnel lens sheet 6 as spreading. Therefore, the inside of the image light 21 is mostly radiated from the fresnel lens 9 to a lenticular lens sheet (not shown). However, as shown in FIG. 7C, the smallest portion inside the incident image light 21 reaches the rising faces 10 instead of fresnel lens faces 9. After reaching the rising faces 10, the image light directly radiates toward the lenticular lens sheet or reflects from the rising faces 10 before radiates from the fresnel lens faces 9. Such image light is called stray light since it may not be perpendicularly incident to the lenticular lens sheet. The stray light mixes noise into the regular image light to degrade the contrast of the projection TV. Known in the art is a technique for installing a light absorbing layer or a light scattering layer on the rising faces in order to restrain the stray light from occurring. (Refer to Japanese Laid-Open Patent Application No. S50-123448, Japanese Laid-Open Patent Application No. S52-143847, Japanese Laid-Open Patent Application No. S62-251701, Japanese Laid-Open Patent Application No. S62-251702 and so on.) However, it is difficult to install the light absorbing layer only on the rising faces, and even if properly installed, the manipulation is troublesome as drawbacks.